U.S. Pat. No. 6,434,408 B1 discloses a system and a method related to improvements in pulse oximetry approaches. In particular, a method for use in a pulse oximetry system which provides a detector output indicative of light absorption by a tissue-under-test at each of a plurality of different light wavelengths is presented, the method comprising:
utilizing said detector output to compute blood analyte indicator values for each of a plurality of measurements, and obtaining a corresponding relative motion estimate value for each of said plurality of measurements; and,
determining whether the corresponding relative motion estimate value for each of said plurality of measurements is within a first predetermined range, wherein for at least one of said plurality of measurements having a corresponding relative motion estimate value within a first predetermined range the corresponding blood analyte indicator value is adjusted utilizing a predetermined adjustment factor that is empirically determined, and wherein said adjusted blood analyte indicator value is employable to obtain a blood analyte concentration value.
The document further discloses several refinements of the method and the system. The document is particularly directed to patient monitoring, such as monitoring a patient's blood oxygen saturation (SpO2). In this connection, pulse oximetry making use of photoplethysmographic approaches can be utilized. For instance, light signals corresponding to two or more different wavelength portions can be employed to non-invasively determine blood components. Basically, blood oxygen saturation measurement can be based on measuring the absorption of oxyhemoglobin (oxygenated hemoglobin) and so-called reduced hemoglobin. Differences in the respective absorption behavior can be indicative of a present SpO2 level. In this connection, it can be exploited that the reduced hemoglobin typically absorbs more light than oxyhemoglobin in a first wavelength portion and, vice versa, that oxyhemoglobin absorbs more light than reduced hemoglobin in a second distinct wavelength portion.
Basically, photoplethysmography is considered a conventional technique which can be used to detect blood volume changes in the tissue of a monitored subject. Conventionally known PPG-approaches include so-called contact PPG devices which can be attached to the skin of the subject of interest, for instance to a finger tip or earlobe. The PPG waveform typically comprises a pulsatile physiological waveform attributable to cardiac synchronous changes in the blood volume with every heart beat. Besides that, the PPG waveform can comprise further embedded information attributable to respiration, oxygen saturation, and to even further physiological phenomena.
Although even standard PPG is considered a basically non-invasive technique, contact PPG requires measurement components (e.g., light sources and photo detectors) which basically have to be attached to the subject's skin. Consequently, standard photoplethysmography still comprises somewhat obtrusive measurements, e.g. via a transceiver unit being firmly fixed to the subject's earlobe or finger tip. Therefore, contact PPG measurement is often experienced as being unpleasant.
Typically, a standard (or: contact) PPG device includes artificial light sources to be directly attached to an indicative surface, e.g., a skin portion, of the subject to be observed. In this manner, reduction or even avoidance of adverse effects is achieved. For instance, potentially disturbing incident radiation caused by other (or: ambient) light sources and undesired subject motion with respect to the light sources can be addressed in this way. Correspondingly, in contact PPG devices also the receiver or detector, e.g. at least one photodiode, is closely fixed to the subject's skin patch of interest. In case the transceiver unit is too firmly fixed to the subject so as to avoid subject movement with respect to the equipment, signal quality can be deteriorated as well, e.g. due to undesired tissue compression.
Recently, remote PPG approaches applying unobtrusive measurements have been introduced. Basically, remote photoplethysmography utilizes light sources or, in general radiation sources, disposed remote from the subject of interest, preferably, for some applications even readily available existing (ambient) light sources rather than defined special-purpose light sources are utilized. For instance, artificial light sources and/or natural light sources can be exploited. Consequently, in remote PPG environments, it has to be expected that due to widely changing illumination conditions, the detected signals generally provide a very small signal-to-noise ratio. Similarly, also a detector, e.g., a camera or at least one photodetector, can be disposed remote from the subject of interest for remote PPG measurements. Therefore, remote photoplethysmographic systems and devices are considered unobtrusive and can be adapted and well suited for everyday application. The field of application may comprise unobtrusive in-patient and out-patient monitoring and even leisure and fitness applications. In this regard, it is considered beneficial that observed subjects can enjoy a certain degree of freedom of movement during remote PPG measurement.
Consequently, compared with standard (obtrusive) photoplethysmography, remote (unobtrusive) photoplethysmography is far more susceptible to distortion and noise. Undesired subject motion with respect to the detector and/or the radiation source can excessively influence signal detection.
In summary, remote PPG is still considered to pose major challenges to signal detection and signal processing. Since the recorded data, such as captured, reflected or emitted electromagnetic radiation (e.g. recorded image frames) always comprises, besides the desired signal to be extracted therefrom, further signal components deriving from overall disturbances, for instance noise due to changing luminance conditions and/or relative motion between the observed subject and the detection sensor, a detailed precise extraction of the desired signals is still considered to pose major problems for existing detection approaches and processing algorithms.
An important field for PPG measurements is the determination of blood oxygen saturation. Contact pulse oximeters typically transmit red and infrared (or, more precisely, in some cases near infrared) light through a vascular tissue of the subject of interest. The respective light portions (R/IR) can be transmitted and detected in an alternating (fast-switching) manner. Given that the respective spectral portions are differently absorbed by oxygenated hemoglobin (HbO2) and reduced hemoglobin (Hb), blood oxygen saturation eventually can be processed. An oxygen saturation (SpO2) estimation algorithm can make use of a ratio of the signals related to the red and the infrared portion. Furthermore, the algorithm can consider a non-pulsatile signal component. Typically, the PPG signal comprises a DC component and a relatively small pulsatile AC component. Furthermore, SpO2 estimation generally involves an empirically derived calibration factor applied to the processed values. Typically, the calibration factor (or, calibration curve) is determined upon reference measurements involving invasive blood oxygen saturation measurements. A calibration factor is required since a PPG device basically detects a ratio of (spectral) signal portions which has to be transferred into a blood oxygen saturation value which typically involves a ratio of HbO2 and Hb. For instance, but not intended to limit the present disclosure, blood oxygen saturation estimation can be based on the following general equation:
                                                        S              p                        ⁢                          O              2                                =                                                    (                                  Hb                  ⁢                  O                                )                            2                                                                        Hb                  ⁢                  O                                2                            +                              H                b                                                    ,                            (        1        )            whereas PPG devices merely mediately detect HbO2 and Hb.